He was elected in June 2004 with 10.93% (881 votes) of the vote and 0.55% of a quota on the first count in a 4 seat electoral area. His victory was one of the closest of the elections, beating Fianna Fáil's Tom Cox by 40 votes.[2]

He worked on various campaigns ranging from Quality Bus Corridors to better sewage treatment and waste management facilities. He has worked on many local committees including the city's Economic Development Group and Environment Strategic Policy Committee, and became Galway's first Green Party Mayor in June 2006. He opened the Galway City Museum to the public. He is a campaigner for the Western Railway Corridor, Light rail for Galway and water quality issues.

Prior to his campaign for City Council, he ran for Dáil Éireann in the Galway West constituency at the 2002 general election receiving 4.43% (2,193 votes) of the vote but was not elected. In February 2005, he was nominated as Galway West's Green Party candidate for the 2007 general election, but he failed to be elected getting 5.49% (3,026 votes) of the vote.

Having been elected to Galway City Council by only 40 votes in 2004, he failed to get elected by just 26 votes in another cliffhanger in the new 5 seat electoral area of Galway City West in what was a very poor election for the Green Party nationally. He got 8.03% (706 votes) of the vote and 0.48% of a quota.[3]

He was elected to the Seanad on 14 December 2009 in a by-election caused by Alan Kelly's election to the European Parliament. Ó Brolchain was nominated as the Green Party candidate and received the backing of coalition partners, Fianna Fáil.[4] The electorate consisted of members of the Dáil and Seanad. He won 119 votes out of 205.